The ethidium cation of the salt ethidium bromide has been previously shown by others to be a sequence specific nucleic acid intercalating agent. We have demonstrated the interaction of gel bound ethidium ion into nucleic acid double helices by a variety of biophysical techniques including fluorescence. Preliminary experiments indicate that crude transfer ribonucleic acid can be fractionated to some degree on this column using elution with a linear sodium chloride gradient. It has also been found that tRNA can be selectively reacted at the X-base with N-hydroxysuccinimide esters of fluorescent compounds. This reaction has been studied as a source of information about the function of this group and as a spectroscopic probe of tRNA structure.